This invention relates to an operation of a steel making electric furnace, and more particularly to a method of incorporating gas blowing from a furnace bottom into melting and refining operations for ordinary steel or alloy by arc heat of electrodes in an electric furnace.
AC arc electric furnaces have been used for melting and refining operations for ordinary steels, alloys or stainless steels. The operation of the electric furnace is basically carried out by charging main scrap raw materials, making an arc directly between the raw materials and electrodes to thereby melt the raw materials, then supplying suitable sub-refining raw materials to provide molten steel of required composition, tilting a converter per each time and tapping the steel and repairing weakened parts of refractory bricks, and repeating scrap charge.
Conventionally, slag-metal in reaction, or the components moving in the steel bath have been controlled by diffusion rate determination. With respect to P and S, for example, they were 2.times.10.sup.-5 cm.sup.2 /s and 4.times.10.sup.-5 cm.sup.2 /s in the molten metal, and they were 4.times.10.sup.-6 cm.sup.2 /s and 2.times.10.sup.-6 cm.sup.2 /s in the slag. Therefore, said reaction and movement were slow, and an arc energy transmission was limited, so that following problems occurred.
(1) There is no room for increasing Fe yield. For example, assuming that total Fe content (T.Fe) in the slag is 20% and the slag amount per product steel is 50 Kg/T, a loss arises that the Fe content of 10 Kg/T is moved into the slag.
(2) Distribution of temperature and components in a layer of slag and metal remaining in the furnace become easily ununiform, and non-molten solids could stay until a refining end.
(3) Since the components moving between the steel and the slag are only slightly diffused from boundary of the slag and the steel the moving speed is easily missed, and since not only (C) but also (Mn), (P), (S) and (O) depend upon the diffusion rate determination, and much time is taken for reaction.
(4) The arc energy is absorbed in the slag layer, and the arc heat is limited narrlowly around the electrodes, and the heat transmission is limited, so that electric power is largely consumed
(5) Alloy iron and deoxidizing agent to be inherently used for reducing the metal are oxidized by the slag, and they are unnecessarily consumed.
As measurements for the above mentioned matters, one method has been adopted that oil blowing auxiliary burners are provided in a furnace wall above a slag line, or oxygen blowing lances are provided instead of the burners. This method is effective for acceleration of melting the scraps above the slag line, however not so under the slag line, and since the steel bath could not be agitated, this method is hardly effective for transmission of the arc energy and for acceleration of metalurgical reaction.
Another method has been proposed, which blows the gas within the furnace from a lower part of the side wall or the furnace bottom in the operation of the electric furnace. But each of them has practical problems.
One of the above methods installs gas blowing pipes at the lower part of the furnace wall corresponding to a cold zone (not opposing parts between electrodes, that is, parts of the furnace wall about the electrodes), and blows gases into the steel bath at low temperature in the cold zone. However, the bath at the low temperature only moves within the furnace, and the melting and refining could not be accelerated so much. Further, this method is not regulated in the proper amount and pressure of blow ing the gas. Since much gas is supplied, the gas using efficiency is inferior.
Another method disposes a plurality (more than 30) of nozzles (single hole) in a concentric circle of the furnace bottom except the parts under the electrodes, and blows the oxygen in a melting period and an inert gas in a refining period. However, since too many nozzles are disposed, the furnace bottom is weakened in strength therearound, and damages of the refractory become extreme, and much time is required for reparing but it results in prolonging the steel making time. There are many dangerous cases in which the steel bath leaks from the gas blowing nozzles. In addition, as the amount and pressure of blowing the gas are not regulated, the an excessive amount of gas and pressure would be used, so that the bath would be chilled or bubbled in the surface, and electric current would be made unstable and consumed a lot, and a basic unit of electric power would be increased.
Each of the above mentioned methods might be expected to a certain extent in the heat conductivity to the cold zone adjacent to the hot zone in the period of melting the scraps. But this is unsatisfied in the heat conductivity to the upper layer of nonuniform scraps which substantially occupy the space within the furnace. So, the shortening of the melting time has naturally been limited.